Why does the colour wheel exist?

[sophiecentaur]

I decided against the "can I really see violet" experiment due to both practical and philosophical difficulties. But I did look at the macaque data in the previously ref'ed paper (Table 1. in http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1192171/pdf/jphysiol00526-0162.pdf) and believe that it indicates a small up-blip for red-cone sensitivity in the far blue meaning that there may be significant differences in the 3-space signals (combined with green response) to either side of "pure" blue. Still would be nice to see the human-Nature paper, but who am I to ask...

Some of this discussion may be off-point due to conflation of magenta and violet -- and whatever happened to "indigo" which was previously touted to be out there beyond violet? Magenta is a _non-spectral_ color and is produced by exciting blue and red cones near both their peaks. Violet is spectral and does -- in theory -- have a specific wavelength somewhat below 400nm. One of my philosophical difficulties with experimenting was that I have no idea how to point to the actual BLUE stripe from a prism in order to say that there is something I see on the other side. I suppose that could be solved with a spectrometer...but I can't get that at the library either.

I also noticed -- finally -- that the two response graphs on the wiki pages (color vision, color space) have different vertical scales. The vision one mumbles something about being linear and seems to be normalized, whereas the space one (which shows the red-blip) is clearly log. So maybe we've been talking about the same thing all along.

As to seeing color change into the IR, the tabular data and wiki graphs both indicate that green and blue sensors level out as the red tails off. So I presume all one sees is a gradual dimming of red rather than a color shift.

Do you not see the difference between a curve that gives the sensitivity of each receptor to a set of wavelengths and a curve that shows the way that human colour vision MATCHES a colour to one which which has been synthesised with three standard phosphors? Try to think out of the box!

@I like serena
Violet is an exceptional 'colour', in that it lies outside the gamut of most phosphor triads. Consequently, it cannot actually be synthesised on a TV screen or by any other normal colour syntheses. It is a bad example because we actually 'see' violet very seldom. The violet we see is mostly a very desaturated magenta and not spectral violet at all. If you try to look at the (spectral) violet in a rainbow, it it very low luminance and is lost in the white / blue background of the clouds /sky. To see a synthesised violet you would need a phosphor that is already violet - you couldn't do it with a normal RGB set of phosphors and 'Ultra Violet' is, by definition, invisible and wouldn't register on any of out sensors.
I assume you accept my argument about the synthesis of Yellow, as an example - so you can see how violet isn't included in any practical system.
Colours that lie outside the gamut of colour TV phosphors are all reproduced as colours lying on the perimeter of the triangle. If you look at crowd scenes, you will notice an awful lot of brightly coloured clothes that appear to be of the same colour (rain jackets at a tennis match, for instance). They aren't all really the same colour in the original scene but the TV system gives them all the same CIE coordinates on the TV screen, which is the best it can do. Violet is never shown, any more than a really saturated orange or yellow.

Also, we really do see the violet when we look at spectral violet. It's not "fooling" our sensors - what we get from the three of them is a combination of three signals that is unique to spectral violet. Violet (and saturated spectral red) are the only two colours that will only match to their own monochromatic wavelength. We can "fool" our eyes pretty well about other spectral colours.

"Fooling" is not the word to apply to the analysis of our eyes. All we do is put lots of spectral patterns into the same 'memory compartment" and group them as the same colour. Like I said already - our eye is not a spectrometer - it just gives us a very crude analysis of the spectra we see, using just three parameters. Nature never knowingly does more than it needs to. What would we do with a spectral plot of everything we saw? Would it be of any more use to us than 'YUV' values?

 

[schip666!]

Do you not see the difference between a curve that gives the sensitivity of each receptor to a set of wavelengths and a curve that shows the way that human colour vision MATCHES a colour to one which which has been synthesised with three standard phosphors? Try to think out of the box!

yah yah, I get the idea that the CIE graph is generated by color-matching experiments and the other one is -- maybe -- the measured sensitivities of each cone type. It did take a bit for me to grok that, but I get it...Chalk it up to language difficulties... But I believe one needs a blue bump in the red receptors response in order to get a real violet sensation and the macaque data indicates that there is such a bump. This is not shown on the putative cone sensitivity graph. My hypothesis is that the graph's Y axis is linear and what we are looking for is down in the noise so would only show up on a log plot.

Further, to my simple mind the easiest way to get to the CIE matching results is to have that blue response bump. Otherwise we'd be having to mix some green into the magentas, no?

Plus I would still like to see some definitive measurements of human cone sensitivity.

I think we are talking about the same thing, just using different terms...

 

[I like Serena]

we actually 'see' violet very seldom. The violet we see is mostly a very desaturated magenta and not spectral violet at all.

That's a good point!

I guess the only place we ever really see violet in practice, is indeed in a rainbow.

So I guess I'm wrong thinking it has a tinge of red in it.

 

[sophiecentaur]

I am so pleased that people are taking this on board at last! I was beginning to feel like Cassandra.

I think the reason for that 'bump' on the red primary matching curve relates to the fact that both red and yellow cone responses extend right over there and it is necessary to exaggerate the amount of red primary presented to the eye for it to notice any difference between the M and L receptor responses - else it would be difficult to differentiate between colours that lay in the blue region of the chart.

Furthermore, there are all those colours that lie on the 'straight bit' of the CIE chart and which are below the central 'white' spot. The only way to match those is to present the eye with high levels of R and B phosphors and little, or no Y phosphor. To represent a good, saturated bluish, magenta you would also need a large contribution of red primary, although the colour you are after is way over on the blue side of the triangle. But you can't draw a simple graph of that which would be along the lines of the one 'with a red bump'.

Now we all have access to computers and colour sliders we can play at producing all sorts of colour matches and get a better clue about just how the CIE diagram and the triangle of primaries work. It can be very 'illuminating' to play with RGB values and see what comes out on your colour control panel.

http://en.wikipedia.org/wiki/CIE_1931_color_space" link gives a good description of things. They make the point that there is a practical limit to the wavelengths usable for the R and B primaries which can be used because our sensitivity is very low at either end of what we actually call 'visible'. We would need kW of output for such phosphors and to no point.

 

[sophiecentaur]

Further, to my simple mind the easiest way to get to the CIE matching results is to have that blue response bump. Otherwise we'd be having to mix some green into the magentas, no?

Not sure I agree with your there, actually. If you added Green primary, you would just get a desaturated Magenta. Although I do believe that, as I have said before, the 'violet' we see on TV is probably just a desaturated magenta. We just forgot what real violet actually looks like 'cos we always see it on TV. (Just like so many people have forgotten what real cooking tastes like!)

 

[sophiecentaur]

Gor blimey strike a light guv'nor. I just looked at the lower, expanded, CIE chart on that link and just look at the actual names of the colours. Take a look at where the name 'violet' appears. (Press the 'expand' button and it's all very readable). Well I never.

 

[I like Serena]

Gor blimey strike a light guv'nor. I just looked at the lower, expanded, CIE chart on that link and just look at the actual names of the colours. Take a look at where the name 'violet' appears. (Press the 'expand' button and it's all very readable). Well I never.

 

[DaveC426913]

At one time, all such information would cost you money. You had to buy books for it or go to a library. There is no justification for complaining that it's not all freely available.

(Aside:
What kind of logic is that?? At one point, a fire department and health care would have cost you money too. This is the 21st century. Standard of lving means there are things we can expect, such as a rich electronic and mostly free cyberspace.)

 

[sophiecentaur]

That's a fair point but the fact that a lot of information is 'free' has nothing to do with morals and rights. It has to do with fact that it is not actually free. The bits of cyberspace that you think you are getting for nothing are funded by the fact that someone pays for the privilege of being able to target you with advertisements. You will notice that the adverts on this very forum reflect the content of the posts.
The difference between 'free' internetting and 'free' fire and health services is that we pay up-front with taxes for one and the other cost is a stealth cost. We have no rights associated with the latter because he who pays the piper calls the tune.
I pay for an email service and do not have to put up with all the garbage on hotmail etc.. If my service is interrupted, then have the right to complain and I do. If hotmail stops for a few hours - only the advertisers can complain.
A subscription service for information can be expected to be better and more useful and exclusive. No one can complain about that.

 

[schip666!]

Not sure I agree with your there, actually. If you added Green primary, you would just get a desaturated Magenta. Although I do believe that, as I have said before, the 'violet' we see on TV is probably just a desaturated magenta. We just forgot what real violet actually looks like 'cos we always see it on TV. (Just like so many people have forgotten what real cooking tastes like!)

That's what I meant. Given our "current understanding" of color -- or colour for thouse of you reading along in europe -- adding green to magenta just desaturates it. If our violet sense is from slightly different responses in red and green cones _all in the same decreasing direction_ it implies (again modulo my simplistic understanding) that one needs green to see violet. However, if there is a red-bump-out-there-in-the-blue we get violet in a way that is congruent with our CIE color matching model.

That fact that they call it "violet" on the chart is part of the language problem. I still want to have a good look at a real color spectrum from a prism or suchlike to remind myself what violet really looks like after all these years of RGB screens. Will probably have to wait for a visit to a science museum or something -- and they may have gotten it wrong anyway.

<RANT>
To continue beating the information availability horse... It's worse then previously described. Most journals actually CHARGE the authors for publication as well as extracting free labor in the form of peer-review, both in exchange for the prestige of appearing in their august pub. Authors retain some copyright so they can put the paper on their website but usually can't print the thing elsewhere. Using the pay-per-view model journals end up enforcing the stratification of disciplines. As in my IEEE society example, if one only does computer-vision one can probably just subscribe to a couple journals and get all one needs. But if you cross a disciplinary boundary you end up hunting through multiple pubs, each for a price, and much of the material you pay for is not relevant...Just one of my a little high-horses.
</RANT>

 

[I like Serena]

That fact that they call it "violet" on the chart is part of the language problem.

Let's not call it "violet", since that is a very ambiguous color (as you can find http://en.wikipedia.org/wiki/List_of_colors#V").
Usually it has red in it, but it can have many different colors.

Let's call it "spectrum violet", so we know what we're talking about.

 

[schip666!]

Let's call it "spectrum violet", so we know what we're talking about.

OK by me. The next time someone actually SEES spectrum violet, report your findings back to us.

 

[sophiecentaur]

There's an evolutionary aspect to all this. There would be little point in an early homo saps having a separate brain pigeon hole for a spectral colour that it doesn't meet very often. The only spectral violet likely to be seen would be in very desaturated form in a rainbow (sitting 'inboard' of blue, not beyond it).Would this deserve a special 'colour' sensation? I don't think so. It would make good design sense to lump it in with some other bluish, desaturated colours. Otoh, we are particularly discriminated about the so-called flesh tones because the colours of our faces are a good way to assess our mood.

 

[I like Serena]

Well, I have seen spectrum red near infrared, which is easy with all the infrared LEDs lying around.
However, IMO this red has a definite reddish look and feel to it.
Where's the difference?

 

[DaveC426913]

Well, I have seen spectrum red near infrared, which is easy with all the infrared LEDs lying around.

Don't forget that bell curve thing works both ways. Some erstwhile "infra-red" LEDs may emit into the far visible red.

 

[DaveC426913]

There's an evolutionary aspect to all this. There would be little point in an early homo saps having a separate brain pigeon hole for a spectral colour that it doesn't meet very often. The only spectral violet likely to be seen would be in very desaturated form in a rainbow (sitting 'inboard' of blue, not beyond it).Would this deserve a special 'colour' sensation? I don't think so. It would make good design sense to lump it in with some other bluish, desaturated colours. Otoh, we are particularly discriminated about the so-called flesh tones because the colours of our faces are a good way to assess our mood.

Why is everyone assuming there is little of this spectral violet around? What makes you think all of nature does not freely reflect in the violet range? It happily reflects in the UV (as witnessed in flowers by birds and bees), so...

 

[schip666!]

There's an evolutionary aspect to all this...Otoh, we are particularly discriminated about the so-called flesh tones because the colours of our faces are a good way to assess our mood.

I can't think of a good evolutionary reason for my hypothesized red-bump-in-the-blue which supposedly supplies color discrimination into the ultra-violet. If it really exists it might be a chemical accident in the red cone. But a nice accidental color none-the-less...

I believe we are most sensitive to shades of green, supposedly to distinguish food sources. Any red gradation sensitivity for facial recognition would have to be a much later development since a) most faces were not "pink-white" to start with; and, b) the social necessity of mood recognition is also a recent thing. Chimps for instance are all about body-language, not subtle facial expression.

But this is all off topic, no? Can we hijack the thread?

 

[I like Serena]

Why is everyone assuming there is little of this spectral violet around? What makes you think all of nature does not freely reflect in the violet range? It happily reflects in the UV (as witnessed in flowers by birds and bees), so...

That's because we don't *know* this is spectrum violet.
We see it, but we can't discern it. Especially since it will be mixed with other colors.

Don't forget that bell curve thing works both ways. Some erstwhile "infra-red" LEDs may emit into the far visible red.

I worked on a machine that relied heavily on infrared leds.
Some people could see whether they were on or off, but I couldn't.
I blamed my eyes for not going into the infrared far enough.
That is, until I took my glasses off!
Then I could *just* see it.

 

[sophiecentaur]

Why is everyone assuming there is little of this spectral violet around? What makes you think all of nature does not freely reflect in the violet range? It happily reflects in the UV (as witnessed in flowers by birds and bees), so...

There IS very little 'spectral' violet around. The violet we see is very de-saturated and nowhere near the envelope of that CIE chart. A rainbow would be the only natural occurrence with loads of scattered white light mixed in. There is a significant difference between the shape of the curved sections of the chart around blue and around red. I think this is probably because there are actually more natural saturated red sources in life (red hot objects at night, for instance).

 

[DaveC426913]

There IS very little 'spectral' violet around.

I don't understand by you say this. Every white object is reflecting loads of violet. Just because we don't see it or can't discern it doesn't mean it's not there.

 

[sophiecentaur]

A component of a perceived colour is not that colour. Of course ther will be some very short wavelengths in all the colours we see when outside in the day. But our eye is not a spectrometer. All we can be aware of is the combination of the outputs of our three sensors at once and we cannot discriminate closer than that. It wasn't until Newton started using prisms that anyone ever experienced near-pure spectral violet entering their eyes. Evolution had already provided us with a mechanism fir interpreting this which is not 'adequate' to analyse the new phenomenon accurately.

 

[Drakkith]

I think sophie is saying that while violet light IS in white light, you won't see the violet because of the mix of other colors causing you to perceive white. In other cases even if you have violet you would probably also have red or green or blue, causing you to not perceive the spectral violet by itself, but as another color.

 

[sophiecentaur]

@Dave
Pinky or black skins actually have very similar chrominance values. The difference is largely related to luminance.

 

[DaveC426913]

A component of a perceived colour is not that colour. Of course ther will be some very short wavelengths in all the colours we see when outside in the day. But our eye is not a spectrometer. All we can be aware of is the combination of the outputs of our three sensors at once and we cannot discriminate closer than that. It wasn't until Newton started using prisms that anyone ever experienced near-pure spectral violet entering their eyes. Evolution had already provided us with a mechanism fir interpreting this which is not 'adequate' to analyse the new phenomenon accurately.

Ok, so you're saying our colour perception didn't evolve to be sensitive to violet, since pure violet (alone) is an uncommon occurrence in nature - unlike pure green and pure red, which are common occurences in their pure form in nature.

 

[sophiecentaur]

I think you are getting near. There is no essential difference between the way we respond to spectral violet (400nm) and spectral green (550nm or so) etc., just the three parameters we get. We very seldom, if ever, see any monochromatic light in nature. When watching colours on a TV system there are never any monochromatic 'colours' produced. A monochromatic phosphor isn't very bright so afaik they aren't used.

 

[sophiecentaur]

I can't think of a good evolutionary reason for my hypothesized red-bump-in-the-blue which supposedly supplies color discrimination into the ultra-violet. If it really exists it might be a chemical accident in the red cone. But a nice accidental color none-the-less...

I believe we are most sensitive to shades of green, supposedly to distinguish food sources. Any red gradation sensitivity for facial recognition would have to be a much later development since a) most faces were not "pink-white" to start with; and, b) the social necessity of mood recognition is also a recent thing. Chimps for instance are all about body-language, not subtle facial expression.

But this is all off topic, no? Can we hijack the thread?

Did you not read the title of the graph with that bump on it? It is not a sensitivity curve. It shows the combinations of primaries that can be used to give a MATCH for a given colour. Have you sussed out the difference?
IMHO that bump is there because we just haven't the ability to distinguish well on the fringe of short wave sensitivity.

 

[schip666!]

Did you not read the title of the graph with that bump on it? It is not a sensitivity curve. It shows the combinations of primaries that can be used to give a MATCH for a given colour. Have you sussed out the difference?
IMHO that bump is there because we just haven't the ability to distinguish well on the fringe of short wave sensitivity.

I don't understand why we are looping on this. I admitted that I figured out the difference and the reasoning behind the CIE graph bump in a long-ago post. However, if you look at the tabular sensitivity data in the previously ref'ed macaque paper you may find that there _is_ a slight increase in sensitivity in the high-blue region for the red cone. In macaques. I don't know for humans. _ALSO_ I believe the two graphs upon which we are cycling, the wiki color-vision and the CIE-color-matching, have different vertical scales where the CIE is log. The log scale would accentuate small differences at the bottom of the range.

The only slightly reasonable way to settle this is to find data for human red cone SENSITIVITY and see if it has a high-blue feature similar to the macaque data. And the only reference I've seen posted here which -- might -- have such data is behind Nature's subscription wall. So...I'm going to try to prevail upon some sciency-friends to hijack that paper and have a look for myself.

In other news, you could be right about the chrominance vs luminance issue in human skin, but I would still argue that the ability to distinguish mood via skin color is newer than could be explained by biological evolution. It might be the other way around...

 

[sophiecentaur]

OK, we do seem to be looping on this but I really couldn't see how the colour sensitivity of some monkey eye cells should have any detailed relevance to the "colour wheel" and human perception - when we already have data both on human perception and matching. Colour TV and film were studied in great detail and a lot of real money was spent in the commercial enterprise of colour reproduction. With respect, the monkey data represents a very small additional contribution (with the greatest respect of course; academic work is always potentially valuable) and would only be of really significant if there were to be an equivalent dertailed set of data on colour matching for monkeys. Bearing in mind that our perception is limited to around 8bits and that a 'just noticeable difference' on the chromaticity chart is around 2%, we don't really need more accurate data in order to put this to bed. As long as we agree that the 'red hump' on Wiki doesn't conflict with the 'non-red hump' data then I can let it rest. IF some monkeys / bees / pit vipers happen to have sensitivity peaks outside or inside the human range, I don't think it needs to interfere with the colour wheel thing. Unfortunate that such a naive term like colour wheel should be at the top of this thread - it should be on an Art / Painting forum - but, fair do's, it got the topic going.

I could rest easy if I thought that people were starting to realize that the colours of monochromatic light are a tiny fraction of what we actually see and that a true, saturated, (monochromatic) source is totally outside the experience of all of us who do not work in a spectrometry Lab. These "pure reds" that people say we see every day are really very impure because they originate from pigments. TV screens or interference filters - all extremely crude sources, with either a broad band spectrum or loads of white light diluting them.

Hanging on to the words "red orange yellow green blue indigo violet" in a discussion about colourimetry is the equivalent to saying "nature abhors a vacuum" when discussing the gas laws.

 

[schip666!]

OK, we do seem to be looping on this but I really couldn't see how the colour sensitivity of some monkey eye cells should have any detailed relevance...

You are absolutely right. I don't care about monkeys either...but it's the only data I've been able to access.

My point, which may be minor or moot, is this: to distinguish spectral-violet as a different "color" from blue we need to get some kind of additional sensory information. This could be just the declining ratios of red and green -- but that implies to me that we could "fake" a sensation of spectral violet by adding a bit of "pure" red and green to a mostly blue signal.

Since we can get a sensation of magenta, which is kinda like violet but not really (at least to my color memory since I haven't seen any good violets lately), by mixing only red with blue it would seem ockhams-razorish that the sensation of violet also has just a bit of red-signal in the mix. Thus my hypothesized and slightly proven-in-monkeys red-bump-in-the-blue argument.

Looking at the Photoshop magentas pallet -- which don't seem to match a nice violet -- somewhat convinces me that there is something else going on, but I would need a good spectral-violet signal for comparison.

Also the easily thrown number of colors distinguishable by the human eye is around 1 million, so the sensors have less than 8 bits of resolution and are probably logarithmic in response...Which is another thing that has always puzzled me: aren't "CRT" displays by nature linear, i.e., twice as much signal makes twice as much light? So how do they get matched to the eye? Are the 1 million stretched over the "extra" 15M colors in an 8+8+8 bit representation by mapping somehow?

Ah well...

 

[sophiecentaur]

You are absolutely right. I don't care about monkeys either...but it's the only data I've been able to access.

My point, which may be minor or moot, is this: to distinguish spectral-violet as a different "color" from blue we need to get some kind of additional sensory information. This could be just the declining ratios of red and green -- but that implies to me that we could "fake" a sensation of spectral violet by adding a bit of "pure" red and green to a mostly blue signal.

R,Gand B signals are not the outputs from your sensors. You need to get your terms defined properly if you want to get this straight. RGB are weightings of three standard primaries for a match with the colour you want to produce. There is loads of data showing the sensitivity of the eye receptors to the spectral colours - wiki shows some of these HML figures on that graph.

Since we can get a sensation of magenta, which is kinda like violet but not really (at least to my color memory since I haven't seen any good violets lately), by mixing only red with blue it would seem ockhams-razorish that the sensation of violet also has just a bit of red-signal in the mix. Thus my hypothesized and slightly proven-in-monkeys red-bump-in-the-blue argument.

The violet that we can actually produce with RGB signals is, indeed, inside the primary triangle and does involve red. Without some red and green, all you would get would be the primary blue (0,0,255). With equal R and B (255,0,255), you get saturated Magenta and adding some G and a bit less than equal R, you get violet(ish) colours, say (100,50 ,255).
All this is not helped by the fact that the primaries are not monochromatic, afaik, so the 'colour' of the blue primary could not actually lie exactly on the curved portion of the CIE chart - it would have to lie inside.

Looking at the Photoshop magentas pallet -- which don't seem to match a nice violet -- somewhat convinces me that there is something else going on, but I would need a good spectral-violet signal for comparison.

How many more times do I have to make this point? The Photoshop pallette consists colours that already have been synethesised with primaries. How could you expect it to have any real validity for colours outside the triangle?

Also the easily thrown number of colors distinguishable by the human eye is around 1 million, so the sensors have less than 8 bits of resolution and are probably logarithmic in response...Which is another thing that has always puzzled me: aren't "CRT" displays by nature linear, i.e., twice as much signal makes twice as much light? So how do they get matched to the eye? Are the 1 million stretched over the "extra" 15M colors in an 8+8+8 bit representation by mapping somehow?

Ah well...

The linearity is, indeed a problem and limits the exposure range of digital cameras. TV pictures and prints have a very limited contrast ratio. A contrasty scene can reveal all sorts of detail to the eye of the person there but the low level stuff is way down in the quantising noise because you are only dealing with a few bits of resolution.

 

[I like Serena]

R, G and B signals are not the outputs from your sensors.

I'd say the perceptual output of our sensors is spectrum red, spectrum violet, and something undefined in the middle.

That is, if only the sensor for red is stimulated, we'd perceive it as red near infrared.
And if only the sensor for blue is stimulated, we'd perceive it as spectrum violet (unless there is a red bump doing something).
But if only the sensor for green is stimulated, we do not know what we would see, since it cannot happen (not without some serious surgery ).

Any combination would be processed by our neural network to be perceived as some color modeled by the CIE diagram.

 

[sophiecentaur]

Aren't you assuming three narrow band analysis curves here? In the extreme, if there were none of that essential overlap, you'd only be aware of three possible colours.